The problem of controlling electrostatic charge is well known in the field of photography. It is also generally known that electrostatic charge can usually be effectively controlled by incorporating an electrically-conductive "antistatic" layer into the film structure. An antistatic layer can be applied to either side of the film base as a subbing layer, that is, beneath the imaging layer or on the side opposite to the imaging layer. An antistatic layer can alternatively be applied as an outer coated layer either over the emulsion layers or on the side of the film base opposite to the emulsion layers (i.e., the backside of the film). For some applications, the antistatic agent can be incorporated into the emulsion layers. Alternatively, the antistatic agent can be directly incorporated into the film base itself. Typically, however, the antistatic layer is employed on the backside of the film and frequently it underlies an abrasion resistant, protective topcoat.
A wide variety of electrically-conductive materials can be incorporated into antistatic layers to produce a wide range of conductivity. These can be divided into two broad groups: (i) ionic conductors and (ii) electronic conductors. In ionic conductors charge is transferred by the bulk diffusion of charged species through an electrolyte. Here the resistivity of the antistatic layer is dependent on temperature and humidity. Antistatic layers containing simple inorganic salts, alkali metal salts of surfactants, ionic conductive polymers, polymeric electrolytes containing alkali metal salts, and colloidal metal oxide sols (stabilized by metal salts), described previously in patent literature, fall in this category. These antistatic layers generally exhibit a substantial loss of antistatic function as a result of exposure to photographic processing solutions.
Antistatic layers containing electronic conductors such as conjugated conducting polymers, conducting carbon particles, crystalline semiconductor particles, amorphous semiconductive fibrils, and continuous semiconducting thin films can be used more effectively than ionic conductors to dissipate static charge since their electrical conductivity is independent of relative humidity and only slightly influenced by ambient temperature. The antistatic properties of such electronic conductors may or may not be affected by photographic processing depending on the particular material. Of the various types of electronic conductors, electrically conducting metal-containing particles, such as semiconducting metal oxides, are particularly effective when dispersed in suitable polymeric film-forming binders in combination with polymeric non-film-forming particles as described in U.S. Pat. Nos. 5,340,676; 5,466,567; 5,700,623. Binary metal oxides doped with appropriate donor heteroatoms or containing oxygen deficiencies have been disclosed in prior art to be useful in antistatic layers for photographic elements, for example, U.S. Pat. Nos. 4,275,103; 4,416,963; 4,495,276; 4,394,441; 4,418,141; 4,431,764; 4,495,276; 4,571,361; 4,999,276; 5,122,445; 5,294,525; 5,382,494; 5,459,021; 5,484,694 and others. Suitable claimed conductive metal oxides include: zinc oxide, titania, tin oxide, alumina, indium oxide, silica, magnesia, zirconia, barium oxide, molybdenum trioxide, tungsten trioxide, and vanadium pentoxide. Preferred doped conductive metal oxide granular particles include antimony-doped tin oxide, fluorine-doped tin oxide, aluminum-doped zinc oxide, and niobium-doped titania. Additional preferred conductive ternary metal oxides disclosed in U.S. Pat. No. 5,368,995 include zinc antimonate and indium antimonate. Other conductive metal-containing granular particles including metal borides, carbides, nitrides and suicides have been disclosed in Japanese Kokai No. JP 04-055,492.
The generation and accumulation of electrostatic charge on film or paper surfaces leads to a variety of problems associated with the manufacture and use of these products. For example, electrostatic charge promotes the attraction of dirt and dust which can lead to these particles being imaged on the film during exposure or printed or projected when these particles are attracted to an already exposed and processed product such as a negative, slide, or motion picture print film. The discharge of accumulated charge during or after the application of the sensitized emulsion layer(s) can produce irregular fog patterns or "static marks" in the emulsion. The static problems have been aggravated by increases in the sensitivity of new emulsions, increases in coating machine speeds, and increases in post-coating drying efficiency. Electrostatic charge can accumulate during winding and unwinding operations, during transport through the coating machines and during finishing operations such as slitting and spooling. Electrostatic charge can also be generated during the use of the finished photographic film product. In an automatic camera, the winding of roll film in an out of the film cartridge, especially in a low relative humidity environment, can result in static charging. Similarly, high speed automated film processing can result in static charge generation. Sheet films (e.g., x-ray films) are especially susceptible to static charging during removal from light-tight packaging.
The use of conductive or antistatic layers on photographic products has greatly minimized many of these abovementioned problems associated with electrostatic charge. However, only very recently has it become recognized that the use of a conductive layer can actually exacerbate some static-related problems. When an electrostatic charge is generated on an insulating surface which overlies a buried conductive layers or is on the side of the film opposite to a conductive layer, the conductive layer is unable to dissipate this surface charge. Instead, the conductive layer can "image" the charge by polar charge formation (that is, the conductive layer is able to draw up an equal, but, opposite charge to that on the surface layer). The formation of this image charge or polar charge within the conductive layer effectively collapses the external field generated by the surface charge so that the field becomes internalized within the film. The presence of an external field could otherwise attract airborne dirt and dust particles to the film surface which can lead to several problems already discussed. In this case then, the presence of the conductive layer and the formation of polar charge helps to eliminate a static-related problem, namely dirt and dust attraction. Referring now to FIG. 1 of the prior art, which schematically illustrates the cross-sectional view of an imaging element such as a photographic film, the polymer substrate 12 is provided with an insulating, image forming layer 10 on its front surface and a conductive layer 14 on its back surface. During conveyance of the imaging element, the image forming layer 10, as a result of contact with dissimilar materials such as rollers or other surfaces, may develop a positive electrostatic charge. The conductive layer 14 then forms an image charge so that the electric field E is internalized within the imaging element.
However, the formation of polar charge can lead to a variety of film sticking problems when the image charge within the conductive layer on one side of the film couples to the (opposite sign) surface charge on the other side of on an adjacent lap of film within a roll or on another sheet of film that it is in contact with. Referring now to FIG. 2 of the prior art, which schematically illustrates this film sticking phenomenon, a film 20 which may be present as a sheet of film or a lap of film on a roll, contains a polymer substrate 26, an insulating, image forming layer 24 on its front surface and a conductive layer 28 on its back surface. Likewise, a film 22, which again may be present as a sheet of film or a lap of film on a roll which is adjacent to film 20, contains a polymer substrate 32, an image forming layer 30, and a conductive layer 34. Both films 20 and 22 have a positive surface charge on imaging layers 24 and 30, respectively, and an image charge in the conductive layers 28 and 34, respectively.
An electrostatic attraction force exists between the positively charged image forming layer 30 of film 22 and the negatively charged conductive layer 28 of film 20. This force of attraction increases as the distance between the two films is decreased and results in the sticking together of these two sheets of film or alternatively two adjacent laps of film. Examples where this film sticking has been observed include sheet films such as graphic arts films, microfiche, and x-ray films that contain a conductive layer whose conductive properties survive film processing. Such films become charged as a result of contact with rollers during film processing and can cause jams in the film processor or difficulties in handling the films after processing.
For motion picture print films containing a conductive layer such as those described in U.S. Pat. No. 5,679,505, film sticking may cause jams in film projectors employing endless loop platter systems such as those described in U.S. Pat. Nos. 4,186,891, 4,208,018, and others. Referring now to FIG. 3 of the prior art, which schematically illustrates this platter sticking problem, an endless loop platter system 30 contains an inner lap of film 32 which is pulled from the core of the film roll and transported along film path 36 to the film projector. The film is rewound onto the outer lap of the film roll via film path 38 as it returns from the projector. As a result of film sticking due to the process already described in FIG. 2, when inner lap 32 is drawn from the core of the roll it may stick to the adjacent lap 34. Film lap 34 may in turn stick to the adjacent outer lap, and so on. Thus multiple laps of film may be pulled simultaneously from the core of the film roll and become jammed in the platter system potentially damaging the projector system, the film, or both.
Increasing the resistivity (or decreasing the conductivity) of a conductive or antistatic layer contained on an imaging element to greater than about 1.times.10.sup.9 .OMEGA./.quadrature. can reduce the tendency for the above sticking problems. However, by increasing the resistivity of the conductive layer one may also significantly reduce overall antistatic protection provided by the conductive layer. In particular, the layer may not be sufficiently conductive to prevent static marking of the film during high speed finishing operations such as film slitting, chopping, or perforating. Although it may be possible to optimize the resistivity of the film so that static protection in finishing operations is provided while film sticking is eliminated, simultaneously achieving both of these attributes is a very difficult challenge.
It is an object of the present invention to provide an improved imaging element which effectively minimizes both film sticking and static marking caused by electrostatic charge.